When did social life start?

Living together with a division of labour in order to ease the 'individual' life is something which is not only found in humans but also in several living beings. So, when did it start? A single cell, say cyanobacteria, was capable of surviving on its own. It could survive in such a stringent condition, decreasing the CO2 content of the atmosphere so that it gradually became amiable for evolution of higher order organisms. In Microbiology, some 'single-cell' organisms have a social life. When in trouble (as in the dearth of nutrition), they come together to form a 'fruiting body' in order to disperse to a place with better nutrition facilities. The amzing fact is that, in this process, a significant part of the population has to sacrifice themselves to form the stalk of the fruiting body. Who are these altruistic fellows? Do they do it on their own or they are forced to sacrifice?
Scientists ivestigate this with various angles; right from investigation olecular mechanisms of signalling for stalk formation to game theoretical aspect f cheating and altruism. Well, if the single cell organism is so complicated, can we realy imagine how much complicated 'our' social life is (at least in terms of scientific queries)? What exactly 'being social' means to us?